Photoacoustic measurement is based on the tendency of molecules in a gas, when exposed to certain wavelengths of radiant energy (e.g. infrared light), to absorb the energy and reach higher levels of molecular vibration and rotation, thereby reaching a higher temperature and pressure within a measurement cell. When the radiant energy striking a gas is amplitude modulated at a known frequency, the resulting fluctuations in energy available for absorption produce corresponding temperature and pressure fluctuations in the gas, which can be measured as an acoustic signal. The amplitude of the acoustic signal is proportional to the intensity of the radiation and the concentration value of the absorbing gas. Such devices are well suited for measuring small concentration values of gases (i.e., in the parts-per-billion range).
Prior art photoacoustic measurement devices have several components in common. In particular, an energy source produces radiant energy which is modulated at a known frequency either thermally (power on/off) or with a chopping device. The modulated energy is provided to a cell containing a gas that absorbs the radiant energy leading to temperature fluctuations in the gas that track the modulation frequency. Temperature is not sensed directly. Rather, pressure fluctuations that accompany the temperature fluctuations are detected by a sensitive microphone in the cell. The microphone output is detected at the modulation frequency, to provide an electrical signal proportional to gas concentration.